A GEPON system is a fiber-optic subscriber system such as an FTTB (Fiber To The Building), or a fiber-optic subscriber access network system that provides a subscriber with an Internet service with a maximum transmission rate of 1 gigabit/second or 2.5 gigabits/second such as an FTTH (Fiber To The Home). The GEPON system comprises a station-side optical line terminal (OLT) installed in a central station, an optical branching device for splitting a transmission line into a maximum of 32 to 64, and a subscriber-side optical network unit (ONU) installed in a subscriber's home. A 1310 nm band wavelength is assigned to an upstream digital data signal which is transmitted from the subscriber-side optical network unit (ONU) to the station-side optical line terminal (OLT). On the other hand, a 1490 nm band wavelength is assigned to a downstream digital data signal or the like (inclusive of a digital voice signal) which is transmitted from the station-side optical line terminal (OLT) to the subscriber-side optical network unit (ONU), and a 1550 nm band wavelength is assigned to a downstream video signal (inclusive of an analog video signal). In this way, it carries out single-core bidirectional optical communication, which transmits the upstream wavelength and downstream wavelength through a single fiber, by using wavelength division multiplexing (WDM) capable of assigning multiple wavelengths.
As for the upstream 1310 nm band optical signal, the GEPON system, using a group called a packet or cell as a unit, controls its transmission timing in such a manner that the packets or cells arrive at the station-side optical line terminal (OLT) at different timings without overlapping each other, thus transmitting them from each subscriber-side optical network unit (ONU). Then, the upstream optical signals from the individual subscriber-side optical network units (ONU) are simply multiplexed with a multiplexer/demultiplexer like an optical splitter, and are supplied to the station-side optical line terminal (OLT).
Accordingly, the GEPON system has a problem in that if a subscriber-side optical network unit (ONU) emits light at timing other than the regular optical signal transmission timing, the station-side optical line terminal (OLT) cannot receive correctly because of a collision with an upstream optical signal of other subscriber-side optical network unit (ONU). In particular, if it makes a light-emission error continuously, it will interfere with communications of all the subscriber-side optical network units (ONU) connected to the same station-side optical line terminal (OLT).
Generally, the transmitter unit of an optical transmitter-receiver used for a subscriber-side optical network unit (ONU) of a GEPON system transmits an optical signal in response to a driving signal such as an input data signal (digital data signal itself to be transmitted) and a pre-bias signal (signal indicating duration in which the subscriber-side optical network unit (ONU) has its own transmission right). As for these data signal and pre-bias signal, although a control LSI such as a PON-LSI (Large Scale Integration) outputs them, if the control LSI goes out of order and if the potential of at least one of the data signal and pre-bias signal is brought to a “High” level fixed state (referred to as High-fixed state from now) or to an indefinite potential state, the optical transmitter-receiver emits light continuously, thereby transmitting the optical signal at erroneous timing.
Thus, Patent Document 1, for example, discloses a light-emission error preventing circuit for preventing the light-emission error of the subscriber-side optical network unit (ONU). The light-emission error preventing circuit, using a photo-detector such as a monitor photodiode for detecting an optical output interruption, detects as a light-emission detecting signal the light emission of a light-emitting element like a laser diode that emits light in response to the driving signal, makes a matching/mismatching decision by comparing the light-emission detecting signal with the driving signal, and makes a light-emission error decision in the case of mismatching.